Example of what we could do to index pip-installed packages

This isn't very efficient, we should use pep 658 metadata which pypi provides
now so we don't have to redownload the wheel. Also we should do fetches
concurrently and asyncio.gather them. But it gets the basic concept.

micropip/_commands/freeze.py

@@ -1,12 +1,110 @@
 import importlib.metadata
 import json
+import sys
 from copy import deepcopy
-from typing import Any
+from importlib.metadata import Distribution
+from typing import TypedDict
 
 from packaging.utils import canonicalize_name
+from packaging.version import Version
 
 from .._compat import REPODATA_INFO, REPODATA_PACKAGES
 from .._utils import fix_package_dependencies
+from ..package_index import query_package
+
+IN_VENV = sys.prefix != sys.base_prefix
+
+
+class PkgEntry(TypedDict):
+    name: str
+    version: str
+    file_name: str
+    install_dir: str
+    sha256: str | None
+    imports: list[str]
+    depends: list[str]
+
+
+def get_pkg_entry_micropip(dist: Distribution, url: str) -> PkgEntry:
+    name = dist.name
+    version = dist.version
+    sha256 = dist.read_text("PYODIDE_SHA256")
+    assert sha256
+    imports = (dist.read_text("top_level.txt") or "").split()
+    requires = dist.read_text("PYODIDE_REQUIRES")
+    if not requires:
+        fix_package_dependencies(name)
+        requires = dist.read_text("PYODIDE_REQUIRES")
+    if requires:
+        depends = json.loads(requires)
+    else:
+        depends = []
+
+    return dict(
+        name=name,
+        version=version,
+        file_name=url,
+        install_dir="site",
+        sha256=sha256,
+        imports=imports,
+        depends=depends,
+    )
+
+
+async def get_pkg_entry_pip(dist: Distribution) -> PkgEntry | None:
+    resp = await query_package(dist.name)
+    ver = resp.releases.get(Version(dist.version), None)
+    if ver is None:
+        return None
+    wheel = next(ver)
+    await wheel.download({})
+    requires = [req.name for req in wheel.requires(set())]
+    return dict(
+        name=dist.name,
+        version=dist.version,
+        file_name=wheel.url,
+        install_dir="site",
+        sha256=wheel.sha256,
+        imports=[],
+        depends=requires,
+    )
+
+
+async def freeze2() -> str:
+    """Produce a json string which can be used as the contents of the
+    ``repodata.json`` lock file.
+
+    If you later load Pyodide with this lock file, you can use
+    :js:func:`pyodide.loadPackage` to load packages that were loaded with :py:mod:`micropip`
+    this time. Loading packages with :js:func:`~pyodide.loadPackage` is much faster
+    and you will always get consistent versions of all your dependencies.
+
+    You can use your custom lock file by passing an appropriate url to the
+    ``lockFileURL`` of :js:func:`~globalThis.loadPyodide`.
+    """
+    packages = deepcopy(REPODATA_PACKAGES)
+    for dist in importlib.metadata.distributions():
+        name = dist.name
+        url = dist.read_text("PYODIDE_URL")
+        if url:
+            pkg_entry = get_pkg_entry_micropip(dist, url)
+        elif IN_VENV:
+            res = await get_pkg_entry_pip(dist)
+            if not res:
+                continue
+            pkg_entry = res
+        else:
+            continue
+
+        packages[canonicalize_name(name)] = pkg_entry
+
+    # Sort
+    packages = dict(sorted(packages.items()))
+    package_data = {
+        "info": REPODATA_INFO,
+        "packages": packages,
+    }
+    return json.dumps(package_data)
 
 
 def freeze() -> str:
@@ -24,32 +122,10 @@ def freeze() -> str:
     packages = deepcopy(REPODATA_PACKAGES)
     for dist in importlib.metadata.distributions():
         name = dist.name
-        version = dist.version
         url = dist.read_text("PYODIDE_URL")
         if url is None:
             continue
-
-        sha256 = dist.read_text("PYODIDE_SHA256")
-        assert sha256
-        imports = (dist.read_text("top_level.txt") or "").split()
-        requires = dist.read_text("PYODIDE_REQUIRES")
-        if not requires:
-            fix_package_dependencies(name)
-            requires = dist.read_text("PYODIDE_REQUIRES")
-        if requires:
-            depends = json.loads(requires)
-        else:
-            depends = []
-
-        pkg_entry: dict[str, Any] = dict(
-            name=name,
-            version=version,
-            file_name=url,
-            install_dir="site",
-            sha256=sha256,
-            imports=imports,
-            depends=depends,
-        )
+        pkg_entry = get_pkg_entry_micropip(dist, url)
         packages[canonicalize_name(name)] = pkg_entry
 
     # Sort